1. Field of Invention
The present invention relates to display systems. The invention is more particularly related to increasing the contrast ratio of display systems. The invention is yet more particularly related to increasing the contrast ratio in projection display systems, including, but not limited to, projections systems using Liquid Crystal on Silicon (LCoS) based light engines.
2. Discussion of Background
When a video display must be physically large and have a high-resolution, a cost effective means to produce the image can be through the use of a projection technology. Amongst the various video projection technologies, the three channel LCoS based approach is a particularly cost effective alternative. A block drawing of the components in a projection television based on conventional LCoS technology is illustrated in FIG. 1.
A challenge faced by all display technologies is to produce an image with a high contrast ratio. There are many approaches to increasing the contrast ratio of the image produced by a LCoS based video projection system. Nonetheless, the current state of the art in LCoS projection is such that further improvement in contrast ratio is desired in even the most advanced systems.
The primary background information necessary to understanding the invention disclosed in this document relates to a display's “gamma curve”. One version of this concept is explained with reference to FIG. 2. This graph charts the relationship between the percentage of incident light that is reflected from a given pixel in a LCoS microdisplay and the shade of gray or bit number assigned to that level of reflectivity. Consider some details of the relationship:                A fully bright pixel corresponds to a shade of gray assigned number 1.        A fully dark (black) pixel corresponds to the shade of gray assigned number 256.        The relationship between the shades of gray assigned numbers between 1 and 256 and the reflectivity of the pixel is not linear. The relationship is called a gamma curve.        Even at the highest bit numbers (the darkest shades of gray), the reflectivity of the pixel is not zero and therefore not completely black.        
This last point is particularly significant because Contrast Ratio=(Reflection in the full bright state)/(Reflection in the full dark state) and even a small residual reflectivity in the dark state results in a dramatic reduction in the contrast ratio. FIG. 2 also defines shades of gray having a bit value below 192 as being “bright” pixels and those having a value above 192 as being “dark” pixels, however, this definition is arbitrary but reasonable and will be utilized in the detailed description of the invention below.